p3: map Brave LLM Context; RRF merge; per-source no_rerank

Live llm/context hits live in grounding.generic plus a URL-keyed
sources map, not a sources array. Fan-in uses reciprocal-rank fusion
instead of cosine. no_rerank keeps that source's own order as a prefix.

internal/config/config.go

@@ -53,14 +53,16 @@ type Search struct {
 }
 
 type SearchSource struct {
-	Enabled bool   `toml:"enabled"`
-	URL     string `toml:"url"`
+	Enabled  bool   `toml:"enabled"`
+	URL      string `toml:"url"`
+	NoRerank bool   `toml:"no_rerank"`
 }
 
 type TavilySource struct {
 	Enabled        bool   `toml:"enabled"`
 	URL            string `toml:"url"`
 	DailyCreditCap int    `toml:"daily_credit_cap"`
+	NoRerank       bool   `toml:"no_rerank"`
 }
 
 func (s Search) RerankOn() bool {